Cochlear implant systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve.
Typically, cochlear implant systems have consisted of essentially two components, an external component commonly referred to as a processor unit and an internal implanted component commonly referred to as a stimulator/receiver unit. Traditionally, both of these components have cooperated together to provide the sound sensation to a user.
The external component may consist of a microphone for detecting sounds, a speech processor that converts the detected sounds, particularly speech, into a coded signal, a power source such as a battery, and an external transmitter antenna.
The coded signal output by the speech processor is transmitted transcutaneously to the implanted stimulator/receiver unit that can be situated within a recess of the temporal bone of the implantee. This transcutaneous transmission occurs via the external transmitter antenna which is positioned to communicate with an implanted receiver antenna provided with the stimulator/receiver unit.
The implanted stimulator/receiver unit traditionally includes a receiver antenna that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlear electrode assembly which applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound.
The commonly accepted method of providing the implanted stimulator with power and information is to transmit RF-power via an inductively coupled antenna coil system. In such a system, the external transmitter coil is usually positioned on the side of an implantee's head directly facing the implanted coil of the stimulator/receiver unit to allow for the transmission of the coded sound signal and power from the speech processor to the implanted unit. Such transmitters usually have a coil formed by a small number of turns of a single or multi-strand wire and a magnet at or near the hub of the coil. The magnet holds the transmitter coil in place due to magnetic attraction with a magnet of the implanted unit.
The implanted magnet can pose problems for those cochlear implant implantees that may be required to undergo magnetic resonance imaging (MRI). In this regard, although studies have indicated that MRI presents no major risk to such implantees, the magnetic fields used in MRI procedures have been shown to exert a torque force on the implanted magnet. This torque force, if significantly large, such as may be the case if a high field strength MRI is undertaken, has the potential to cause undesirable consequences such as dislodgement of the magnet from its casing as well as discomfort to the implantee. There is also the potential for significant distortion of the image obtained by MRI due to the presence of the magnet in the implantee's head, which may significantly negate the usefulness of the process.